1. Field of the Invention
The present invention is directed generally to a digital signal processing method and system, and more particularly, to a digital signal processing method and system suited to convert AC signals of an electric power system into digital signals and detect an electric quantity of the AC signals from the converted digital signals.
This invention also relates generally to a signal processing system for obtaining a physical quantity of effective values of input signals of voltages and currents which are inputted from an electric power system, an electric power system digital control system and a digital relay system which utilize this signal processing system, and more particularly, to those suitable for digital signal processing to digitally converting the input signals.
2. Description of the Prior Art
As disclosed in Japanese Patent Laid-Open Publication NO. 121420/1988 or stated from p. 12 onwards in "A Report of the Electric Engineering Association", No. 12, Vol. 105, a known convectional system of this type is employed for an electric power system digital protective relay. This type of system comprises an input unit, processing unit, a settling unit and an output unit. Incorporated into this input unit is a digital signal processor including an analog filter for eliminating high frequencies, a sample hold circuit, a multiplexer, and A/D converter and a buffer. This system adopts the following construction. Higher harmonic components overlapped with fundamental harmonics are eliminated from analog input signals by means of the analog filter. Output signals of the analog filter are sampled with a period of 600 Hz, thereby converting the analog signals into digital signals. Impedances or magnitudes of a voltage or current of the electric power system are obtained from the digital signals. The relay is thus operated.
Based on the conventional techniques given above, the analog filter is capable of eliminating the higher harmonic components overlapped with the analog input signals and disturbance noises as well. No consideration is, however, given to noises caused in the portions subsequent to the analog filter, such noises including, e.g., the disturbance noises and those incidental to errors in quantization of the A/D converter. This in turn causes a disadvantage of generating an error in arithmetic value of digital data due to those noises. The following is a description in greater detail. A sampling frequency is set at 600 Hz. In a frequency band, the noises are produced concomitantly with the quantization errors and the disturbance noises when effecting sampling. This frequency band is overlapped with a pass band (frequency band of analog input signal) of the analog filter. In consequence, the analog input signals can not be separated from the noises concomitant with the disturbance noises and the quantization errors. It follows that these noises cause arithmetic errors. For this reason, it is impossible to sufficiently enhance a performance of resolving power of the A/D converter.
Known as an electric power control system which utilizes this type of system is a voltage/reactive power control system stated on pp. 1200-1201 in a report of "The 946th National Meeting of The Electric Engineering Association" in 1987.
When driving this type of electric power control system, a common practice is detection of effective values of the AC signals of the electric power system. On the occasion of the detection of effective values of the AC signals, the following method has hitherto been adopted. The AC signals are inputted to the filter for attenuating the higher harmonic components thereof. A frequency of the AC signal is detected from an output signal of the filter. A gain of the output signal of the filter is corrected based on the detected frequency. On the basis of the corrected signal, the effective value of the AC signal is calculated. According to this method, even when the frequency of the AC signal fluctuates, the gain is corrected to keep the filter output signal at a constant level. It is therefore possible to obtain the effective values corresponding to the fluctuations in frequency.
There arise, however, the following problems inherent in the prior art. Sufficient concerns are, as will be explained later, not paid to characteristics of the input filter. If the frequency of the AC signal fluctuates, the electric quantity of the AC signals can not accurately be obtained in some cases.
(1) Since the input filter is composed of an analog filter, there are caused a scatter in elements, a deterioration in property due to a change with a passage of time and an inter-channel scatter.
(2) The input signal is sampled at 600 Hz (or 720 Hz: electrical angle of 30.degree. of fundamental harmonic). It is therefore required that an intricate algorithm be executed to obtain the frequency with a high accuracy. Besides, a multiplicity of arithmetic units are required to be added.
(3) A gain characteristic of the input filter is fixed. Hence, when the frequency of the input signal fluctuates, it is also required that the gain be corrected.
(4) When the frequency of the AC signal fluctuates, the fundamental harmonic of the fluctuated AC signal can not sufficiently be attenuated. This results in an increase in detection error of the effective value.